1. Field of the Invention
The present invention relates to apparatus and methods for reliably determining when a semiconductor wafer is securely clamped in place. More specifically, the invention relates to apparatus and methods that use ultrasonic techniques to reliably determine whether or not a semiconductor wafer is securely clamped in place on a support member.
2. Discussion of the Background
Plasma processing of silicon wafers to transfer a pattern of an integrated circuit from the photolithographic mask to the silicon, or to deposit dielectric or conductive films on the substrate, have become standard methods in the industry. In conventional plasma processors, the silicon wafer being processed is held in close proximity to the wafer chuck, ordinarily by electrostatic force. This system is quite effective in holding the wafer securely to the chuck during processing, permitting good heat transfer between the wafer and the other components in the processing system.
However, all modem manufacturing plasma processors use automatic robotic systems to load the wafer on and off of the wafer chuck. It is essential, therefore, that the status of the clamping between the wafer and the wafer chuck be confirmed after the wafer has been loaded onto the wafer chuck and before it is removed by the loading arm. Failure of clamping after loading the wafer onto the wafer chuck will result in poor quality and uniformity of the process, resulting in poor yields and poor quality of the finished devices.
During deposition of metal on a silicon wafer to form conductive paths that are the electrical interconnects in an integrated circuit, for example, large amounts of electrical energy are delivered to the process chamber to transfer the metal ions from the source of metal to the wafer. As the metal is deposited, energy is delivered to the wafer and if the wafer is not cooled, heat may damage the electrical devices. To keep the wafer cool and to promote formation of the desired metallurgical compounds on the surface of the wafer, the wafer is electrostatically clamped to a chilled surface and a gas is introduced behind the wafer to enhance heat transfer from the wafer to the chilled surface
Because the gas behind the wafer is pressurized to enhance heat transfer, if the wafer is not securely clamped before the gas is introduced behind the wafer, the wafer will float on the cushion of gas, away from its position on the chilled surface. Thus, it is desirable to verify that the wafer is properly clamped before introducing the gas behind the wafer, and to maintain a secure clamp as long as pressurized gas is present behind the wafer.
When the processing is complete, the electrostatic potential holding the wafer to the wafer chuck is turned off. However, residual electrostatic charges may inhibit release of the wafer. In this case, when the robotic system attempts to remove the wafer from the wafer chuck, the wafer may be broken. This is catastrophic, because not only is the valuable wafer lost, it is usually necessary to do a complete tear down and clean out of the processing chamber, costing valuable time and manpower as well as lost production time on the equipment.
Therefore, it is desirable to continuously monitor the status of the clamping between the wafer and wafer chuck.
One alternative method for detecting the presence of a wafer and whether the wafer is satisfactorily clamped is by measuring the capacitance between the wafer and the surface on which the wafer is clamped. The capacitance is measured by injecting a sample RF signal onto an electrode under the wafer and measuring the intensity of the sample frequency RF on a second electrode under the wafer.
A second alternative method for detecting whether the wafer is satisfactorily clamped is by measuring the flow necessary to maintain a pressure of gas under the wafer. By using very small flows to produce a very slight pressure under the wafer, it may be possible to determine the quality of the clamp of the wafer prior to applying gas pressure under the wafer. Similarly, this technique may be used to determine whether the wafer has been de-clamped from the surface. This technique, however, is not useful for monitoring the clamp status during processing of the wafer since it relies on the opportunity to adjust the pressure or flow of gas behind the wafer. Also, variations in the surface of the wafer may result in variations in flow even though the wafer is securely clamped to the surface.
U.S. Pat. No. 5,271,274 (Khuri-Yakub et al.) discloses a method using ultrasonic acoustic waves to determine the presence and thickness of films on a substrate. The echo of the ultrasonic wave or the phase of the echo is used to measure the thickness of deposited films on a substrate such as a silicon wafer. U.S. Pat. No. 6,019,000 (Stanke et al.) utilizes ultrasonic acoustic waves to perform in-situ measurement of deposition on reactor chamber members. This system also permits the determination of the degree of erosion of chamber members. Both patents utilize reflection of the ultrasonic waves from the surfaces and interfaces between the members and films to determine the presence and thickness of the films. However, neither patent solves the problems of monitoring the status of the clamping between a wafer and a wafer chuck, and of determining when the wafer has been completely released by electrostatic forces holding it to the wafer chuck. It is to fulfill these needs, among others, that the present invention is directed.